1. Field of the Invention
The present invention relates to a thin film magnetic memory device and a method of fabricating the same, and more particularly to a thin film magnetic memory device having a redundant configuration for repairing a defective memory cell and a method of fabricating the same.
2. Description of the Background Art
As a memory device capable of storing data in a nonvolatile manner with low power consumption, attention is being paid to an MRAM (Magnetic Random Access Memory) device. The MRAM device is a memory device for storing data in a nonvolatile manner by using a plurality of thin film magnetic materials formed on a semiconductor integrated circuit. Each of the thin film magnetic materials can be accessed at random.
Particularly, in recent years, it has been announced that the performance of an MRAM device is dramatically improved by using a thin film magnetic material utilizing a magnetic tunnel junction (MTJ) as a memory cell. An MRAM device including memory cells each having a magnetic tunnel junction is disclosed in the following Literature 1 to 3.
(Literature 1)
Roy Scheuerlein and six others, xe2x80x9cA 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and, FET Switch in each Cellxe2x80x9d, (U.S.A.), 2000 IEEE ISSCC Digest of Technical Papers, TA7.2
(Literature 2)
M. Durlam and five others, xe2x80x9cNonvolatile RAM based on Magnetic Tunnel Junction Elementsxe2x80x9d, (U.S.A.), 2000 IEEE ISSCC Digest of Technical Papers, TA7. 3
(Literature 3)
Peter K. Naji and four others, xe2x80x9cA 256 kb 3.0V 1T1MTJ Nonvolatile Magnetoresistive RAMxe2x80x9d, (U.S.A.), 2001 IEEE ISSCC Digest of Technical Papers, TA7.6
FIG. 16 is a schematic diagram showing the configuration of a memory cell having a magnetic tunnel junction (hereinafter, also simply referred to as xe2x80x9cMTJ memory cellxe2x80x9d).
Referring to FIG. 16, an MTJ memory cell has a tunneling magneto-resistance element TMR in which electric resistance changes according to the level of stored data and an access element ATR for forming a path of a sense current Is passing through tunneling magneto-resistance element TMR when data is read. Since access element ATR is formed representatively by a field effect transistor, access element ATR will be also referred to as an access transistor ATR hereinafter. Access transistor ATR is connected between tunneling magneto-resistance element TMR and a fixed voltage (ground voltage Vss).
For the MTJ memory cell, a write word line WWL for instructing writing of data, a read word line RWL for reading data, and a bit line BL as a data line for transmitting an electric signal corresponding to the level of stored data in a data reading/writing operation are disposed.
FIG. 17 is a conceptual diagram for describing reading of data from the MTJ memory cell.
Referring to FIG. 17, tunneling magneto-resistance element TMR has a ferromagnetic layer (hereinafter, also simply referred to as xe2x80x9cfixed magnetic layerxe2x80x9d) FL having a fixed predetermined magnetization direction and a ferromagnetic layer (hereinafter, also simply referred to as xe2x80x9cfree magnetic layerxe2x80x9d) VL magnetized in the direction according to a magnetic field applied from the outside. Between fixed magnetic layer FL and free magnetic layer VL, a tunneling barrier layer (tunnel film) TB made by an insulating film is provided. Free magnetic layer VL is magnetized in the same direction as fixed magnetic layer FL or in the direction opposite to fixed magnetic layer FL in accordance with the level of storage data to be written. By fixed magnetic layer FL, tunneling barrier TB, and free magnetic layer VL, a magnetic tunnel junction is formed.
At the time of reading data, in response to activation of read word line RWL, access transistor ATR is turned on. It enables sense current Is to be passed through a current path constructed by bit line BL, tunneling magneto-resistance element TMR, access transistor ATR, and ground voltage Vss.
Electric resistance of tunneling magneto-resistance element TMR changes according to the relative relation between the magnetization direction of fixed magnetic layer FL and the magnetization direction of free magnetic layer VL. Concretely, when the magnetization direction of fixed magnetic layer FL and that of free magnetic layer VL are the same (parallel), as compared with the case where the magnetization directions are opposite (anti-parallel) to each other, the electric resistance of tunneling magneto-resistance element TMR is lower.
Therefore, by magnetizing free magnetic layer VL in one of the two kinds of directions in accordance with storage data, a voltage change occurring in tunneling magneto-resistance element TMR by sense current Is varies according to the level of the storage data. For example, after precharging bit line BL to a predetermined voltage, sense current Is is passed to tunneling magneto-resistance element TMR, and by detecting the voltage of bit line BL, data stored in the MTJ memory cell can be read.
FIG. 18 is a conceptual diagram for describing an operation of writing data to the MTJ memory cell.
Referring to FIG. 18, at the time of writing data, read word line RWL is made inactive and access transistor ATR is turned off. In this state, a data write current for magnetizing free magnetic layer VL in the direction according to write data is passed to write word line WWL and bit line BL. The magnetization direction of free magnetic layer VL is determined by the data write current flowing in write word line WWL and the data write current flowing in bit line BL.
FIG. 19 is a conceptual diagram showing the relation between the data write current at the time of writing data to the MTJ memory cell and the magnetization direction of the tunneling magneto-resistance element.
Referring to FIG. 19, a horizontal axis H (EA) indicates a magnetic field applied in the direction of a magnetization easy axis (EA) in free magnetic layer VL in tunneling magneto-resistance element TMR. On the other hand, a vertical axis H (HA) indicates a magnetic field acting in the direction of a magnetization hard axis (HA) in free magnetic layer VL. The magnetic fields H (EA) and H (HA) correspond to two magnetic fields generated by the current passing through bit line BL and the current passing through write word line WWL.
In the MTJ memory cell, the fixed magnetization direction of fixed magnetic layer FL is along the magnetization easy axis of free magnetic layer VL, and free magnetic layer VL is magnetized in parallel with (in the same direction as) or in anti-parallel (opposite) with fixed magnetic layer FL along the magnetization easy axis direction in accordance with the level of the stored data (xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d). In the specification, the electric resistance of tunneling magneto-resistance element TMR corresponding to the two kinds of magnetization directions of free magnetic layer VL is expressed by Rmax and Rmin (where Rmax greater than Rmin). The MTJ memory cell can store one-bit data (xe2x80x9c1xe2x80x9d or xe2x80x9c0xe2x80x9d) in correspondence with the two kinds of magnetization directions of free magnetic layer VL.
The magnetization direction of free magnetic layer VL can be newly rewritten only in the case where the sum of magnetic fields H (EA) and H (HA) applied reaches the outside of the asteroid characteristic curve shown in the diagram. In other words, when the data write magnetic field applied has an intensity corresponding to the inside area of the asteroid, characteristic curve the magnetization direction of free magnetic layer VL does not change.
As shown by the asteroid characteristic curve, by applying a magnetic field in a magnetization hard axis direction to free magnetic layer VL, a magnetization threshold value necessary to change the magnetization direction along the magnetization easy axis can be decreased.
In the case where the operating point at the time of writing data is designed as shown in the example of FIG. 19, in an MTJ memory cell to which data is to be written, a data write magnetic field in the magnetization easy axis direction is designed so that its intensity becomes HWR. Specifically, the value of a data write current passed to bit line BL or write word line WWL is designed so that data write magnetic field HWR is obtained. Generally, data write magnetic field HWR is expressed by the sum of a switching magnetic field HSW necessary to switch the magnetization direction and a margin amount AH (HWR=HSW+xcex94H).
In order to rewrite data stored in the MTJ memory cell, that is, the magnetization direction of tunneling magneto-resistance element TMR, a data write current of a predetermined level or higher has to be passed to both of write word line WWL and bit line BL. By the data write current, free magnetic layer VL in tunneling magneto-resistance element TMR is magnetized in the direction parallel to or opposite (anti-parallel) to fixed magnetic layer FL in accordance with the direction of the data write magnetic field along magnetization easy axis (EA). The magnetization direction once rewritten in tunneling magneto-resistance element TMR, that is, the data stored in the MTJ memory cell is held in a nonvolatile manner until new data writing is executed.
A method of stably damaging a desired tunnel film which is generally as thin as a few nm in a tunneling magneto-resistance element TMR is disclosed in U.S. Pat. No. 6,324,093. Concretely, a tunnel film in a tunneling magneto-resistance element TMR in an MTJ memory cell whose tunnel film is to be damaged (hereinafter, also referred to as xe2x80x9cmemory cell to be damagedxe2x80x9d) is damaged to irreversibly fix data stored in the MTJ memory cell, and the device is used as a ROM (Read-Only Memory). In the specification, irreversible fixing of data stored in an MTJ memory cell is also referred to as destructive writing.
Generally, in a memory device, normal operations such as data reading and data writing operations are executed on the basis of program information externally stored in a nonvolatile manner.
Typically, information used for controlling a redundancy configuration for repairing a defective memory cell by replacement with a spare memory cell is stored as program information. In the redundancy configuration, at least a defect address for specifying a defective memory cell has to be stored as program information.
In a conventional memory device (such as a DRAM (Dynamic Random-Access Memory)), the program information is programmed by disconnecting (blowing out) a fuse element with a laser beam or the like.
In an MRAM device as well, a configuration of performing programming by using a fuse element in order to store program information externally is considered.
However, in the configuration of making programming by blowing out a fuse element for replacing a defective memory cell, special equipment such as a trimmer dedicated to laser blowing is necessary. Consequently, time and cost consumed for the programming process increase.
Moreover, the programming process carried out by blowing out a fuse element is executed in a wafer state. Therefore, for example, in a memory device in which a defect address corresponding to a defective memory cell detected in a wafer state is programmed and which is packaged and formed as a product, it is difficult to deal with a defective memory cell which appears later, so that deterioration in the yield occurs.
An object of the present invention is to provide a thin film magnetic memory device to/from which program information can be efficiently and stably stored/read by using the small number of memory cells.
The present invention is summarized as follows. A thin film magnetic memory device includes: a memory array in which a plurality of memory cells each magnetically storing data are arranged in a matrix; and a program circuit for storing information used for at least one of an operation of reading data and an operation of writing data from/to the plurality of memory cells. The program circuit includes a plurality of program cells each for storing program data constructing the information, and each of the memory cells and the program cells includes a magnetic storing part having first and second electric resistances in correspondence with two magnetization directions respectively. The program circuit further includes: a driver circuit for irreversibly fixing electric resistance in the magnetic storing part in one of the plurality of program cells to a third electric resistance with physical destruction; and a sense driver circuit capable of sensing which one of the first and second electric resistances is provided for the magnetic storing part in one of the plurality of program cells in a first mode. The sense driver circuit can sense whether any one of the first or second electric resistances, or third electric resistance is provided for the magnetic storing part in one of the plurality of program cells in a second mode. The first electric resistance is larger than the second electric resistance, and the second electric resistance is larger than the third electric resistance.
Therefore, main advantages of the present invention are that program data can be stored on a bit unit basis, the electric resistance of the magnetic storing part in a selected program cell can be irreversibly fixed by physical destruction, and the electric resistance of the magnetic storing part can be determined in the first and second modes. In such a manner, the thin film magnetic memory device to/from which program data can be efficiently and stably stored/read by using the small number of memory cells can be realized.
According to another aspect of the present invention, there is provided a method of fabricating a thin film magnetic memory device including a plurality of memory cells each for magnetically storing data, including: a repair determining step of determining whether a device can be repaired or not on the basis of a result of a wafer test; a program fixing step which is executed after the repair determining step and irreversibly stores information for repairing the device obtained by the wafer test into a program circuit for the device which is determined to be repairable in the repair determining step; and an assembly step executed after the program fixing step. The program circuit includes a plurality of program cells each magnetically storing program data used for programming the information, each of the program cells in the program circuit has a magnetic storing part for storing data when being magnetized in one of two directions, and electric resistance of the magnetic storing part in each of the program cells in which the program data is stored is fixed by a physical destructive operation in the program fixing step. Therefore, a main advantage of the present invention is that by performing the program fixing step before the assembly step, data in the magnetic storing part can be prevented from being lost in the assembly step and subsequent steps.
According to further another aspect of the present invention, there is provided a method of fabricating a thin film magnetic memory device including a plurality of memory cells each for magnetically storing data, including: a repair determining step of determining whether a device can be repaired or not on the basis of a result of a wafer test; an assembly step executed for the device determined to be repairable in the repair determining step; and a program fixing step which is executed after the assembly step and irreversibly stores information for repairing the device obtained by the wafer test into a program circuit. The program circuit includes a plurality of program cells each magnetically storing program data used for programming the information, each of the program cells in the program circuit has a magnetic storing part for storing data when being magnetized in one of two directions, and electric resistance of the magnetic storing part in each of the program cells in which the program data is stored is fixed by a physical destructive operation in the program fixing step.
Therefore, an another advantage of the present invention is that, by performing the program fixing step after the assembly step, the possibility of repairing a defective which occurs in the assembly step or the like can be increased.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.